The present invention relates to a method for purifying a thermoplastic norbornene resin. The present invention further relates to a substrate of a magnetic recording medium using such a purified thermoplastic norbornene resin. The invention also relates to a magnetic recording medium using such a substrate, and a method for manufacturing such a magnetic recording medium. The magnetic recording medium is installed in an external recording device of a computer or the other magnetic digital data-recording device.
With increasing capacity of recording devices using magnetic recording medium, efforts have been made to decrease the flying height of the magnetic head in order to upgrade the recording density. The flying height of a magnetic head requires a magnetic recording medium with extremely smooth and high precision surface. For example, a conventional substrate made of non-magnetic metal, such as aluminum, needs high precision machining.
An example of a manufacturing method of a conventional substrate of a magnetic recording medium made of non-magnetic metal and a magnetic recording medium using such a substrate is described in the following.
A non-magnetic substrate is generally made of a blank material that is prepared by rolling a heated and molten metal, annealing, and cutting into specified dimensions. The blank material is subjected to inner-and outer-diameter treatment and lapping to improve surface precision. To improve surface hardness, Nixe2x80x94P plating layer, having thickness of 13 xcexcm, is then formed on the blank material. The surface of the Nixe2x80x94P layer is polished to surface roughness Ra=10 xc3x85, and the polished surface is finally lapped using diamond slurry. In a contact start and stop (CSS) zone of the thus obtained substrate, laser zone texture is formed having bump height of 190 xc3x85 and bump density of radial 30 xcexcmxc3x97circumferential 30 xcexcm. Finally, high precision cleaning is conducted on the substrate to obtain a substrate for a magnetic recording medium.
On the thus obtained substrate, a chromium under-layer 500 xc3x85 thick, a magnetic layer of Co-14Cr-4Ta 300 xc3x85 thick, and a carbon protective layer 80 xc3x85 thick are sequentially formed by DC sputtering method. The sputtered surface is tape varnished. Then, a fluorine-containing lubricant layer, 20 xc3x85 thick, is formed by dip-coating or spin-coating method, obtaining a magnetic recording medium.
A conventional method for manufacturing a substrate of a magnetic recording medium and a conventional method for manufacturing a magnetic recording medium are becoming more and more complicated, as the density requirements have increased in recent years. In addition, it is also demanded to provide a magnetic recording medium with less cost than usual while maintaining high functionality of the medium. To compromise these conflicting requirements, a new type of magnetic recording medium has been proposed that uses a substrate made of plastic material.
A method of manufacturing a plastic substrate for a magnetic recording medium by molding and forming a CSS zone simultaneously with the molding provides high productivity and thus industrially advantageous resulting in magnetic recording media with lower cost.
A plastic substrate for a magnetic recording medium is manufactured by injection-molding resin pellets, for example. The molding process easily generates protrusion or recess defects with a few micron size on the substrate surface, which is a different situation from a metal substrate and a ceramic substrate such as glass substrate. This is a serious problem in a substrate for a magnetic recording medium, in which high surface precision is essential.
Producing a magnetic recording medium by using a plastic substrate including such defects may cause failure of read and write by a head or head crash during seeking operation.
Further, a magnetic recording medium produced by using such a plastic substrate may cause blistering on the substrate surface when the substrate is repeatedly left in an environment of high temperature and high humidity and an environment of low temperature and low humidity. The blistering lowers environmental reliability.
Material for a plastic substrate of a magnetic recording medium may be selected from a polycarbonate resin and a poly(methyl methacrylate). A thermoplastic norbornene resin may also be used for a plastic substrate. The thermoplastic norbornene resin exhibits excellent characteristics including high heat resistance, low hygroscopicity, and geometry stability, thus, provides a favorable substrate for a magnetic recording medium. However, the thermoplastic norbornene resin also involves the above-described common problems of a plastic substrate.
It is an object of the present invention to provide a method for purifying a thermoplastic norbornene resin.
It is another object of the present invention to provide a plastic substrate of a magnetic recording medium having scarce surface defects and exhibiting excellent environmental reliability using a thermoplastic norbornene resin purified by such a purifying method.
It is still another object of the invention to provide a magnetic recording medium using such a plastic substrate.
It is further object of the present invention to provide a method for manufacturing such a magnetic recording medium.
The present invention provides a magnetic recording medium holding such a high precision surface that has scarce surface defect even after heat shock test at high temperature and high humidity or low temperature and low humidity as well as at normal temperature and normal humidity.
The inventors of the present invention made numerous studies to solve the above-described problems and found that the objects can be achieved by injection-molding using pellets of thermoplastic norbornene resin that is purified to less than a certain value of impurity concentration. The present invention has been accomplished based on the finding.
The first aspect of the present invention involves a purifying method of a thermoplastic norbornene resin, in which organic impurities, ionic impurities, metallic impurities, and particles are eliminated by cleaning liquid. The cleaning liquid in the invention is 2-propanol (hereinafter also referred to as xe2x80x9cIPAxe2x80x9d) or a mixed solvent of 2-propanol and water. The mixed solvent is a mixture of 2-propanol and water with mixing ratio 2-propanol:water of from 1:1 to 5:1 in volume. Impurity concentration in a resin purified by a purification method according to the invention is favorably as follows: organic impurities, not more than 30 ppb; ionic impurities, not more than 5 ppb; metallic impurities, not more than 5 ppb. The above-mentioned organic impurities may include hydrocarbon impurities, deteriorated antioxidant, and oxidized and deteriorated resin component. Favorable concentration of these impurities are as follows: hydrocarbon impurities, not more than 20 ppb; deteriorated antioxidant, not more than 5 ppb; oxidized and deteriorated resin component, not more than 5 ppb.
The second aspect of the invention involves a plastic substrate for a magnetic recording medium. The plastic substrate is produced by injection-molding a thermoplastic norbornene resin that is purified by an above-described purifying method of the first aspect of the invention. Protrusion defects and recess defects on the substrate surface with the diameter of 1 xcexcm or more are present preferably in a number of not more than 100 per surface.
The third aspect of the invention involves a magnetic recording medium comprising a plastic substrate of the second aspect of the invention; and a magnetic layer, a protective layer and a liquid lubricant layer sequentially formed on the plastic substrate. Advantageously, the magnetic recording medium does not generate any blister having a diameter of not less than 1 xcexcm and a height of not less than 0.1 xcexcm when the medium is left under an environmental condition of 60xc2x0 C. and 80% RH, or xe2x88x9240xc2x0 C. and 10% RH, or a condition combining these conditions.
The fourth aspect of the invention involves a method of manufacturing a magnetic recording medium comprising steps of purifying a thermoplastic norbornene resin, and forming a plastic substrate of a magnetic recording medium by injection-molding the purified thermoplastic norbornene resin; and a step for depositing a magnetic layer, a protective layer, and a liquid lubricant layer sequentially formed on the plastic substrate, wherein the step for purifying the resin comprises a process for cleaning the thermoplastic norbornene resin in a cleaning liquid selected from 2-propanol and a mixed solvent composed of 2-propanol and pure water.
Advantageously, the mixed solvent has a mixing ratio of 2-propanol to pure water of from 1:1 to 5:1 in volume.